Golf ball

ABSTRACT

A golf ball  2  includes an inner core  4 , an outer core  6 , a mid layer  8 , a cover  10 , and dimples  12 . A diameter D1, a volume V1, a hardness H1o at a central point, a boundary inside hardness H1in, and a hardness difference De1 of the inner core  4 ; a volume V2 (mm 3 ) and a hardness difference De2 of the outer core  6 ; a thickness Tm and a hardness Hm of the mid layer  8 ; and a thickness Tc and a hardness Hc of the cover  10  meet the following mathematical formulas. 
       1.0&lt; V 2/ V 1&lt;7.0 
         De 2− De 1&lt;0
 
       600&lt;( H 1 o*H 1in)*( D 1/2)/2&lt;1000 
       620&lt; Tc*Hc*Hm/Tm &lt;900 
     A dimple pattern of each hemisphere of the golf ball  2  includes three units rotationally symmetrical to each other.

This application claims priority on Patent Application No. 2015-248108 filed in JAPAN on Dec. 21, 2015. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to golf balls. Specifically, the present invention relates to golf balls that include a core having a two-layer structure, a mid layer, and a cover and further have a plurality of dimples on the surface thereof.

Description of the Related Art

The greatest interest to golf players concerning golf balls is flight distance. In particular, golf players place importance on flight distances upon shots with a driver. The initial speed, the spin rate, and the launch angle of a golf ball when the golf ball is hit with a golf club are referred to as three initial elements. The flight distance of the golf ball is influenced by the three initial elements. The higher the initial speed of the ball is, the larger the flight distance is. An appropriate spin rate and an appropriate launch angle also contribute to flight distance.

There are various proposals for improvement of flight performance that focuses on the three initial elements. JPH11-206920 discloses a golf ball including an inner core and an outer core. Such golf balls having two core layers are also disclosed in JP2003-190331 (US2003/0130065), JP2006-289065 (US2006/0229143), JP2007-190382 (US2007/0259739), JPH10-328326 (U.S. Pat. No. 6,468,169), JPH10-328328 (U.S. Pat. No. 6,248,027), JP2000-60997 (U.S. Pat. No. 6,251,031), and JP2009-219871 (US2008/0161132).

Golf balls have a large number of dimples on the surfaces thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. This phenomenon is referred to as “turbulization”. Due to the turbulization, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulization promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. Excellent dimples efficiently disturb the air flow. The excellent dimples produce a long flight distance.

There have been various proposals for dimples. JP2009-172192 (US2009/0191982) discloses a golf ball on which dimples are randomly arranged. The dimple pattern of the golf ball is referred to as a random pattern. The random pattern can contribute to the flight performance of the golf ball. JP2012-10822 (US2012/0004053) also discloses a golf ball having a random pattern.

JP2007-175267 (US2007/0149321) discloses a dimple pattern in which the number of units in a high-latitude region is different from the number of units in a low-latitude region. JP2007-195591 (US2007/0173354) discloses a dimple pattern in which the number of the types of dimples in a low-latitude region is larger than the number of the types of dimples in a high-latitude region. JP2013-153966 (US2013/0196791) discloses a dimple pattern having a high dimple density and small variation in dimple size.

Golf players' requirements for flight distance have been escalated. An object of the present invention is to provide a golf ball having excellent flight performance upon hitting with a driver.

SUMMARY OF THE INVENTION

A golf ball according to the present invention includes an inner core, an outer core positioned outside the inner core, a mid layer positioned outside the outer core, and a cover positioned outside the mid layer. A diameter D1 (mm), a volume V1 (mm³), a hardness H1o (Shore C) at a central point, a boundary inside hardness H1in (Shore C), and a difference De1 between the hardness H1in and the hardness H1o of the inner core; a volume V2 (mm³) and a difference De2 between a surface hardness H2s (Shore C) and a boundary outside hardness H2out (Shore C) of the outer core; a thickness Tm (mm) and a hardness Hm (Shore D) of the mid layer; and a thickness Tc (mm) and a hardness Hc (Shore D) of the cover meets the following mathematical formulas (1.1) to (1.4),

1.0<V2/V1<7.0  (1.1),

De2−De1<0  (1.2),

600<(H1o+H1in)*(D1/2)/2<1000  (1.3), and

620<Tc*Hc*Hm/Tm<900  (1.4)

The golf ball has a plurality of dimples on a surface thereof. A ratio So of a sum of areas of the dimples relative to a surface area of a phantom sphere of the golf ball is equal to or greater than 81.0%. A ratio Rs of a number of the dimples each having a diameter of equal to or greater than 9.60% but equal to or less than 10.37%, of a diameter of the golf ball, relative to a total number of the dimples, is equal to or greater than 50%. A dimple pattern of each hemisphere of the golf ball includes three units that are rotationally symmetrical to each other. A dimple pattern of each unit includes two small units that are mirror-symmetrical to each other. The golf ball meets the following mathematical formula (2.1).

Rs≧−2.5*So+273  (2.1)

When the golf ball according to the present invention is hit with a driver, the spin rate is low, and the initial speed of the ball is high. Furthermore, with the golf ball, turbulization is promoted by the dimples. By the appropriate three initial elements and the appropriate aerodynamic characteristic, with the golf ball, a large flight distance is obtained upon a shot with a driver.

Preferably, the golf ball meets the following mathematical formula.

2.0<V2/V1<6.0

Preferably, the golf ball meets the following mathematical formula.

700<(H1o+H1in)*(D1/2)/2<900

Preferably, the golf ball meets the following mathematical formula.

640<Tc*Hc*Hm/Tm<800

Preferably, the golf ball meets the following mathematical formula (2.2).

Rs≧−2.5*So+278  (2.2)

Preferably, the golf ball meets the following mathematical formula (2.3).

Rs≧−2.5*So+283  (2.3)

Preferably, a ratio Rs' of a number of the dimples each having a diameter of equal to or greater than 10.10% but equal to or less than 10.37%, of the diameter of the golf ball, relative to the total number of the dimples, is equal to or greater than 50%. Preferably, the golf ball meets the following mathematical formula (2.4).

Rs′≧−2.2*So+245  (2.4)

Preferably, the golf ball meets the following mathematical formula (2.5).

Rs′≧−2.2*So+252  (2.5)

Preferably, a depth of a deepest part of each dimple from a surface of the phantom sphere is equal to or greater than 0.10 mm but equal to or less than 0.65 mm.

Preferably, a total volume of the dimples is equal to or greater than 450 mm³ but equal to or less than 750 mm³.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a golf ball according to one embodiment of the present invention;

FIG. 2 is an enlarged plan view of the golf ball in FIG. 1;

FIG. 3 is a front view of the golf ball in FIG. 2;

FIG. 4 is a partially enlarged cross-sectional view of the golf ball in FIG. 1;

FIG. 5 is a graph showing a relationship between a ratio So and a ratio Rs;

FIG. 6 a graph showing a relationship between the ratio So and a ratio Rs′;

FIG. 7 is a plan view of a golf ball according to Example 2 of the present invention;

FIG. 8 is a front view of the golf ball in FIG. 7;

FIG. 9 is a plan view of a golf ball according to Comparative Example 4; and

FIG. 10 is a front view of the golf ball in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based on preferred embodiments with appropriate reference to the drawings.

A golf ball 2 shown in FIG. 1 includes a spherical inner core 4, an outer core 6 positioned outside the inner core 4, a mid layer 8 positioned outside the outer core 6, and a cover 10 positioned outside the mid layer 8. The golf ball 2 has a plurality of dimples 12 on the surface thereof. Of the surface of the golf ball 2, a part other than the dimples 12 is a land 14. The golf ball 2 includes a paint layer and a mark layer on the external side of the cover 10 although these layers are not shown in the drawing. The golf ball 2 may include another layer between the outer core 6 and the mid layer 8. The golf ball 2 may include another layer between the mid layer 8 and the cover 10.

The golf ball 2 preferably has a diameter of equal to or greater than 40 mm but equal to or less than 45 mm. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is more preferably equal to or less than 44 mm and particularly preferably equal to or less than 42.80 mm. The diameter of the golf ball 2 according to the present embodiment is 42.70 mm. Thirty points on the land 14 are selected at random. A diameter at each of the points as an end is measured. The diameter of the golf ball 2 is calculated by averaging these diameters.

The golf ball 2 preferably has a weight of equal to or greater than 40 g but equal to or less than 50 g. In light of attainment of great inertia, the weight is more preferably equal to or greater than 44 g and particularly preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is particularly preferably equal to or less than 45.93 g.

The inner core 4 is formed by crosslinking a rubber composition. Examples of preferable base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. In light of high initial speed of the ball, polybutadienes are preferable. When a polybutadiene and another rubber are used in combination, it is preferred if the polybutadiene is a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and particularly preferably equal to or greater than 80% by weight. A polybutadiene in which the proportion of cis-1,4 bonds is equal to or greater than 80% is particularly preferable.

The rubber composition of the inner core 4 preferably includes a co-crosslinking agent. Preferable co-crosslinking agents in light of high initial speed of the ball are monovalent or bivalent metal salts of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Examples of preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. In light of resilience performance, zinc acrylate and zinc methacrylate are particularly preferable.

The rubber composition may include a metal oxide and an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. They both react with each other in the rubber composition to obtain a salt. The salt serves as a co-crosslinking agent. Examples of preferable α,β-unsaturated carboxylic acids include acrylic acid and methacrylic acid. Examples of preferable metal oxides include zinc oxide and magnesium oxide.

In light of high initial speed of the ball, the amount of the co-crosslinking agent per 100 parts by weight of the base rubber is preferably equal to or greater than 10 parts by weight and particularly preferably equal to or greater than 15 parts by weight. In light of low spin rate upon a shot with a driver, this amount is preferably equal to or less than 40 parts by weight and particularly preferably equal to or less than 35 parts by weight. As described in detail later, a low spin rate can achieve a large flight distance upon a shot with a driver.

Preferably, the rubber composition of the inner core 4 includes an organic peroxide. The organic peroxide serves as a crosslinking initiator. The organic peroxide contributes to the resilience performance of the golf ball 2. Examples of suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. An organic peroxide with particularly high versatility is dicumyl peroxide.

In light of high initial speed of the ball, the amount of the organic peroxide per 100 parts by weight of the base rubber is preferably equal to or greater than 0.1 parts by weight, more preferably equal to or greater than 0.3 parts by weight, and particularly preferably equal to or greater than 0.5 parts by weight. In light of low spin rate, this amount is preferably equal to or less than 3.0 parts by weight, more preferably equal to or less than 2.8 parts by weight, and particularly preferably equal to or less than 2.5 parts by weight.

Preferably, the rubber composition of the inner core 4 includes an organic sulfur compound. Organic sulfur compounds include naphthalenethiol compounds, benzenethiol compounds, and disulfide compounds.

Examples of naphthalenethiol compounds include 1-naphthalenethiol, 2-naphthalenethiol, 4-chloro-1-naphthalenethiol, 4-bromo-1-naphthalenethiol, 1-chloro-2-naphthalenethiol, 1-bromo-2-naphthalenethiol, 1-fluoro-2-naphthalenethiol, 1-cyano-2-naphthalenethiol, and 1-acetyl-2-naphthalenethiol.

Examples of benzenethiol compounds include benzenethiol, 4-chlorobenzenethiol, 3-chlorobenzenethiol, 4-bromobenzenethiol, 3-bromobenzenethiol, 4-fluorobenzenethiol, 4-iodobenzenethiol, 2,5-dichlorobenzenethiol, 3,5-dichlorobenzenethiol, 2,6-dichlorobenzenethiol, 2,5-dibromobenzenethiol, 3,5-dibromobenzenethiol, 2-chloro-5-bromobenzenethiol, 2,4,6-trichlorobenzenethiol, 2,3,4,5,6-pentachlorobenzenethiol, 2,3,4,5,6-pentafluorobenzenethiol, 4-cyanobenzenethiol, 2-cyanobenzenethiol, 4-nitrobenzenethiol, and 2-nitrobenzenethiol.

Examples of disulfide compounds include diphenyl disulfide, bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide, bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(4-cyanophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide, bis(2-cyano-5-bromophenyl)disulfide, bis(2,4,6-trichlorophenyl)disulfide, bis(2-cyano-4-chloro-6-bromophenyl)disulfide, bis(2,3,5,6-tetrachlorophenyl)disulfide, bis(2,3,4,5,6-pentachlorophenyl)disulfide, and bis(2,3,4,5,6-pentabromophenyl)disulfide.

In light of high initial speed of the ball, the amount of the organic sulfur compound per 100 parts by weight of the base rubber is preferably equal to or greater than 0.1 parts by weight and particularly preferably equal to or greater than 0.2 parts by weight. In light of low spin rate, this amount is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight. Two or more organic sulfur compounds may be used in combination.

The rubber composition of the inner core 4 may include a filler for the purpose of specific gravity adjustment and the like. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the inner core 4 is accomplished. The rubber composition may include various additives, such as sulfur, a carboxylic acid, a carboxylate, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, and the like, in an adequate amount. The rubber composition may include crosslinked rubber powder or synthetic resin powder.

The inner core 4 preferably has a diameter D1 of equal to or greater than 15.0 mm. With the golf ball 2 that includes the inner core 4 having a diameter D1 of equal to or greater than 15.0 mm, spin upon a shot with a driver is suppressed. In this respect, the diameter D1 is more preferably equal to or greater than 18.0 mm and particularly preferably equal to or greater than 20.0 mm. From the standpoint that the outer core 6 can have a sufficient thickness, the diameter D1 is preferably equal to or less than 32.0 mm, more preferably equal to or less than 29.0 mm, and particularly preferably equal to or less than 27.0 mm.

The inner core 4 preferably has a volume V1 of equal to or greater than 1700 mm³. With the golf ball 2 that includes the inner core 4 having a volume V1 of equal to or greater than 1700 mm³, spin upon a shot with a driver is suppressed. In this respect, the volume V1 is more preferably equal to or greater than 3000 mm³ and particularly preferably equal to or greater than 4200 mm³. From the standpoint that the outer core 6 can have a sufficient volume V2, the volume V1 is preferably equal to or less than 17000 mm³, more preferably equal to or less than 13000 mm³, and particularly preferably equal to or less than 10300 mm³.

The inner core 4 has a weight of preferably equal to or greater than 10 g but equal to or less than 30 g. The temperature for crosslinking the inner core 4 is equal to or higher than 140° C. but equal to or lower than 180° C. The time period for crosslinking the inner core 4 is equal to or longer than 10 minutes but equal to or shorter than 60 minutes.

In the golf ball 2, the difference De1 between a hardness H1o at the central point of the inner core 4 and a boundary inside hardness H1in of the inner core 4 is great. The inner core 4 having the great difference De1 has a so-called outer-hard/inner-soft structure. When the golf ball 2 including the inner core 4 is hit with a driver, the spin rate is low. When the golf ball 2 including the inner core 4 is hit with a driver, a high launch angle is obtained.

Upon a shot with a driver, an appropriate trajectory height and appropriate flight duration are required. With the golf ball 2 that achieves a desired trajectory height and desired flight duration at a high spin rate, the run after landing is short. With the golf ball 2 that achieves a desired trajectory height and desired flight duration at a high launch angle, the run after landing is long. In light of flight distance, the golf ball 2 that achieves a desired trajectory height and desired flight duration at a high launch angle is preferable. The inner core 4 having an outer-hard/inner-soft structure can contribute to a high launch angle and a low spin rate as described above. The golf ball 2 including the inner core 4 has excellent flight performance.

In light of flight performance, the difference De1 is preferably equal to or greater than 5, more preferably equal to or greater than 8, and particularly preferably equal to or greater than 10. In light of ease of producing the inner core 4, the difference De1 is preferably equal to or less than 40 and particularly preferably equal to or less than 30. Preferably, in the inner core 4, the hardness gradually increases from the central point thereof toward the surface thereof.

In light of high initial speed of the ball, the central hardness H1o is preferably equal to or greater than 40, more preferably equal to or greater than 45, and particularly preferably equal to or greater than 50. In light of low spin rate, the hardness H1o is preferably equal to or less than 80, more preferably equal to or less than 75, and particularly preferably equal to or less than 70.

The hardness H1o is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). The hardness scale is pressed against the central point of the cross-section of a hemisphere obtained by cutting the golf ball 2. The measurement is conducted in the environment of 23° C.

In light of low spin rate, the boundary inside hardness H1in is preferably equal to or greater than 60, more preferably equal to or greater than 65, and particularly preferably equal to or greater than 70. In light of durability of the golf ball 2, the hardness H1in is preferably equal to or less than 85, more preferably equal to or less than 80, and particularly preferably equal to or less than 78.

The hardness H1in is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). The hardness scale is pressed against a cross-section of the golf ball 2 that has been cut into two halves. The hardness scale is pressed against a point separated by 1 mm radially inward from the boundary between the inner core 4 and the outer core 6. The measurement is conducted in the environment of 23° C.

In the inner core 4, a value Va calculated by the following mathematical formula exceeds 600 and is less than 1000.

Va=(H1o+H1in)*(D1/2)/2

In other words, the inner core 4 meets the following mathematical formula (1.3).

600<(H1o+H1in)*(D1/2)/2<1000  (1.3)

The value Va is approximate to the integral of a hardness distribution of the inner core 4. With the golf ball 2 that includes the inner core 4 having a value Va of exceeding 600, the spin rate upon a shot with a driver is low. With the golf ball 2 that includes the inner core 4 having a value Va of less than 1000, the initial speed of the ball upon a shot with a driver is high. The low spin rate and the high initial speed of the ball achieve a large flight distance. In this respect, the golf ball 2 more preferably meets the following mathematical formula.

700<(H1o+H1in)*(D1/2)/2<900

In other words, the value Va preferably exceeds 700 and is less than 900.

The outer core 6 is formed by crosslinking a rubber composition. The composition can include the base rubber described above for the inner core 4.

The rubber composition of the outer core 6 preferably includes a co-crosslinking agent. Preferable co-crosslinking agents in light of high initial speed of the ball are monovalent or bivalent metal salts of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Examples of preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. In light of high initial speed of the ball, zinc acrylate and zinc methacrylate are particularly preferable.

The rubber composition may include a metal oxide and an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. They both react with each other in the rubber composition to obtain a salt. The salt serves as a co-crosslinking agent. Examples of preferable α,β-unsaturated carboxylic acids include acrylic acid and methacrylic acid. Examples of preferable metal oxides include zinc oxide and magnesium oxide.

In light of high initial speed of the ball, the amount of the co-crosslinking agent per 100 parts by weight of the base rubber is preferably equal to or greater than 25 parts by weight, more preferably equal to or greater than 30 parts by weight, and particularly preferably equal to or greater than 35 parts by weight. In light of feel at impact, this amount is preferably equal to or less than 55 parts by weight, more preferably equal to or less than 50 parts by weight, and particularly preferably equal to or less than 45 parts by weight.

Preferably, the rubber composition of the outer core 6 includes an organic peroxide. The organic peroxide serves as a crosslinking initiator. The organic peroxide contributes to the resilience performance of the golf ball 2. The rubber composition of the outer core 6 can include the organic peroxide described above for the inner core 4.

In light of high initial speed of the ball, the amount of the organic peroxide per 100 parts by weight of the base rubber is preferably equal to or greater than 0.1 parts by weight, more preferably equal to or greater than 0.3 parts by weight, and particularly preferably equal to or greater than 0.5 parts by weight. In light of feel at impact, this amount is preferably equal to or less than 3.0 parts by weight, more preferably equal to or less than 2.8 parts by weight, and particularly preferably equal to or less than 2.5 parts by weight.

Preferably, the rubber composition of the outer core 6 includes an organic sulfur compound. The rubber composition can include the organic sulfur compound described above for the inner core 4.

In light of high initial speed of the ball, the amount of the organic sulfur compound per 100 parts by weight of the base rubber is preferably equal to or greater than 0.1 parts by weight and particularly preferably equal to or greater than 0.2 parts by weight. In light of feel at impact, this amount is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight.

The rubber composition of the outer core 6 may include a filler for the purpose of specific gravity adjustment and the like. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the outer core 6 is accomplished. The rubber composition may include various additives, such as sulfur, a carboxylic acid, a carboxylate, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, and the like, in an adequate amount. The rubber composition may include crosslinked rubber powder or synthetic resin powder.

The outer core 6 preferably has a diameter D2 of equal to or greater than 37.0 mm. The golf ball 2 that includes the outer core 6 having a diameter D2 of equal to or greater than 37.0 mm has a high initial speed upon a shot with a driver. In this respect, the diameter D2 is more preferably equal to or greater than 37.5 mm and particularly preferably equal to or greater than 38.0 mm. From the standpoint that the mid layer 8 and the cover 10 can have sufficient thicknesses, the diameter D2 is preferably equal to or less than 40.5 mm, more preferably equal to or less than 40.0 mm, and particularly preferably equal to or less than 39.5 mm.

The outer core 6 preferably has a volume V2 of equal to or greater than 18000 mm³. The golf ball 2 that includes the outer core 6 having a volume V2 of equal to or greater than 18000 mm³ has a high initial speed upon a shot with a driver. In this respect, the volume V2 is more preferably equal to or greater than 19500 mm³ and particularly preferably equal to or greater than 21000 mm³. From the standpoint that the mid layer 8 and the cover 10 can have sufficient thicknesses, the volume V2 is preferably equal to or less than 29000 mm³, more preferably equal to or less than 27000 mm³, and particularly preferably equal to or less than 26000 mm³. In the present embodiment, the volume V2 is calculated by subtracting the volume of the inner core 4 from the volume of a sphere consisting of the inner core 4 and the outer core 6.

The outer core 6 has a weight of preferably equal to or greater than 10 g but equal to or less than 30 g. The temperature for crosslinking the outer core 6 is equal to or higher than 140° C. but equal to or lower than 180° C. The time period for crosslinking the outer core 6 is equal to or longer than 10 minutes but equal to or shorter than 60 minutes.

In the golf ball 2, the difference De2 between a surface hardness H2s and a boundary outside hardness H2out of the outer core 6 is preferably equal to or greater than −2 but equal to or less than 2. A hardness distribution of the outer core 6 is close to being flat. In the outer core 6, the energy loss upon hitting with a driver is low. The golf ball 2 that includes the outer core 6 has a high initial speed upon a shot with a driver. The outer core 6 can contribute to the flight performance of the golf ball 2. In light of flight performance, the difference De2 is preferably equal to or greater than −1 but equal to or less than 1. The difference De2 may be zero.

The difference between a Shore C hardness at a point having a highest hardness and a Shore C hardness at a point having a lowest hardness in the zone from the boundary between the inner core 4 and the outer core 6 to the surface of the outer core 6 is preferably equal to or less than 5, more preferably equal to or less than 4, and particularly preferably equal to or less than 3.

In light of high initial speed of the ball, the boundary outside hardness H2out is preferably equal to or greater than 60, more preferably equal to or greater than 70, and particularly preferably equal to or greater than 75. In light of feel at impact, the hardness H2out is preferably equal to or less than 90, more preferably equal to or less than 87, and particularly preferably equal to or less than 85.

The hardness H2out is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). The hardness scale is pressed against a cross-section of the golf ball 2 that has been cut into two halves. The hardness scale is pressed against a point separated by 1 mm radially outward from the boundary between the inner core 4 and the outer core 6. The measurement is conducted in the environment of 23° C.

In light of spin suppression upon a shot with a driver, the surface hardness H2s is preferably equal to or greater than 70, more preferably equal to or greater than 72, and particularly preferably equal to or greater than 74. In light of durability of the golf ball 2, the hardness H2s is preferably equal to or less than 90, more preferably equal to or less than 88, and particularly preferably equal to or less than 86.

The hardness H2s is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). The hardness scale is pressed against the surface of the outer core 6. The measurement is conducted in the environment of 23° C.

A core having a small difference De2 can be obtained by a two-stage crosslinking process. Specifically, in a first crosslinking process, a rubber composition is crosslinked at a predetermined temperature. In a second crosslinking process, the rubber composition is crosslinked at a temperature higher than the temperature in the first crosslinking process.

The ratio (V2/V1) between the volume V2 of the outer core 6 and the volume V1 of the inner core 4 meets the following mathematical formula (1.1).

1.0<V2/V1<7.0  (1.1)

In other words, the ratio (V2/V1) exceeds 1.0 and is less than 7.0. The golf ball 2 having a ratio (V2/V1) of exceeding 1.0 has a high initial speed upon a shot with a driver. The golf ball 2 having a ratio (V2/V1) of less than 7.0 has a low spin rate upon a shot with a driver. The high initial speed and the low spin rate of the ball achieve a large flight distance. In this respect, the golf ball 2 more preferably meets the following mathematical formula.

2.0<V2/V1<6.0

The golf ball 2 particularly preferably meets the following mathematical formula.

2.5<V2/V1<5.0

In the golf ball 2, the hardness difference De2 in the outer core 6 and the hardness difference De1 in the inner core 4 meet the following mathematical formula (1.2).

De2−De1<0  (1.2)

With the golf ball 2 that meets the above mathematical formula, both a high initial speed and a low spin rate are achieved upon a shot with a driver. In this respect, the golf ball 2 more preferably meets the following mathematical formula.

De2−De1<−5

The golf ball 2 particularly preferably meets the following mathematical formula.

De2−De1<−10

The mid layer 8 is positioned between the outer core 6 and the cover 10. The mid layer 8 is formed from a thermoplastic resin composition. Examples of the base polymer of the resin composition include ionomer resins, polyesters, polyamides, polyurethanes, polyolefins, and polystyrenes. Ionomer resins are particularly preferable. Ionomer resins are highly elastic. The golf ball 2 that includes the mid layer 8 including an ionomer resin has excellent resilience performance upon a shot with a driver.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 70% by weight, and particularly preferably equal to or greater than 85% by weight.

Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferable binary copolymer includes 80% by weight or more but 90% by weight or less of an α-olefin, and 10% by weight or more but 20% by weight or less of an α,β-unsaturated carboxylic acid. The binary copolymer has excellent resilience performance. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. A preferable ternary copolymer includes 70% by weight or more but 85% by weight or less of an α-olefin, 5% by weight or more but 30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% by weight or more but 25% by weight or less of an α,β-unsaturated carboxylate ester. The ternary copolymer has excellent resilience performance. For the binary copolymer and the ternary copolymer, preferable α-olefins are ethylene and propylene, while preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. A particularly preferable ionomer resin is a copolymer formed with ethylene and acrylic acid. Another particularly preferable ionomer resin is a copolymer formed with ethylene and methacrylic acid.

In the binary copolymer and the ternary copolymer, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymium ion. The neutralization may be carried out with two or more types of metal ions. Particularly suitable metal ions in light of resilience performance and durability of the golf ball 2 are sodium ion, zinc ion, lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7329”, and “Himilan AM7337”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont de Nemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured by ExxonMobil Chemical Corporation. Two or more ionomer resins may be used in combination.

The resin composition of the mid layer 8 may include a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer includes a polystyrene block as a hard segment, and a soft segment. A typical soft segment is a diene block. Examples of compounds for the diene block include butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferable. Two or more compounds may be used in combination.

Examples of styrene block-containing thermoplastic elastomers include styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenated SBS, hydrogenated SIS, and hydrogenated SIBS. Examples of hydrogenated SBS include styrene-ethylene-butylene-styrene block copolymers (SEBS). Examples of hydrogenated SIS include styrene-ethylene-propylene-styrene block copolymers (SEPS). Examples of hydrogenated SIBS include styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of resilience performance of the golf ball 2, the content of the styrene component in the styrene block-containing thermoplastic elastomer is preferably equal to or greater than 10% by weight, more preferably equal to or greater than 12% by weight, and particularly preferably equal to or greater than 15% by weight. In light of feel at impact, the content is preferably equal to or less than 50% by weight, more preferably equal to or less than 47% by weight, and particularly preferably equal to or less than 45% by weight.

In the present invention, styrene block-containing thermoplastic elastomers include an alloy of an olefin and one or more members selected from the group consisting of SBS, SIS, SIBS, SEBS, SEPS, and SEEPS. The olefin component in the alloy is presumed to contribute to improvement of compatibility with another base polymer. The alloy can contribute to the resilience performance of the golf ball 2. An olefin having 2 to 10 carbon atoms is preferable. Examples of suitable olefins include ethylene, propylene, butene, and pentene. Ethylene and propylene are particularly preferable.

Specific examples of polymer alloys include trade names “RABALON T3221C”, “RABALON T3339C”, “RABALON SJ4400N”, “RABALON SJ5400N”, “RABALON SJ6400N”, “RABALON SJ7400N”, “RABALON SJ8400N”, “RABALON SJ9400N”, and “RABALON SR04”, manufactured by Mitsubishi Chemical Corporation. Other specific examples of styrene block-containing thermoplastic elastomers include trade name “Epofriend A1010” manufactured by Daicel Chemical Industries, Ltd., and trade name “SEPTON HG-252” manufactured by Kuraray Co., Ltd.

In light of feel at impact, the proportion of the styrene block-containing thermoplastic elastomer to the entire base polymer is preferably equal to or greater than 1% by weight and particularly preferably equal to or greater than 2% by weight. In light of spin suppression upon a shot with a driver, the proportion is preferably equal to or less than 15% by weight, more preferably equal to or less than 10% by weight, and particularly preferably equal to or less than 5% by weight.

The resin composition of the mid layer 8 may include a polyamide. With the golf ball 2 that includes the mid layer 8 including a polyamide, the spin upon a shot with a driver is suppressed. Specific examples of polyamides include polyamide 6, polyamide 11, polyamide 12, polyamide 66, and polyamide 610. In light of versatility, polyamide 6 is preferable.

In light of spin suppression, the proportion of the polyamide to the entire base polymer is preferably equal to or greater than 5% by weight, more preferably equal to or greater than 10% by weight, and particularly preferably equal to or greater than 15% by weight. In light of feel at impact, the proportion is preferably equal to or less than 50% by weight, more preferably equal to or less than 45% by weight, and particularly preferably equal to or less than 40% by weight.

The resin composition of the mid layer 8 may include a filler for the purpose of specific gravity adjustment and the like. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The resin composition may include powder of a metal with a high specific gravity. Specific examples of metals with a high specific gravity include tungsten and molybdenum. The amount of the filler is determined as appropriate so that the intended specific gravity of the mid layer 8 is accomplished. The resin composition may include a coloring agent, crosslinked rubber powder, or synthetic resin powder. When the hue of the golf ball 2 is white, a typical coloring agent is titanium dioxide.

The mid layer 8 preferably has a hardness Hm of equal to or greater than 55. The golf ball 2 that includes the mid layer 8 having a hardness Hm of equal to or greater than 55 has a low spin rate upon a shot with a driver. The mid layer 8 can contribute to the flight performance of the golf ball 2. In this respect, the hardness Hm is more preferably equal to or greater than 58 and particularly preferably equal to or greater than 61. In light of feel at impact, the hardness Hm is preferably equal to or less than 80, more preferably equal to or less than 75, and particularly preferably equal to or less than 72.

The hardness Hm of the mid layer 8 is measured according to the standards of “ASTM-D 2240-68”. The hardness Hm is measured with a Shore D type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). For the measurement, a sheet that is formed by hot press, is formed from the same material as that of the mid layer 8, and has a thickness of about 2 mm is used. Prior to the measurement, a sheet is kept at 23° C. for two weeks. At the measurement, three sheets are stacked.

The mid layer 8 has a thickness Tm of preferably equal to or greater than 0.3 mm but equal to or less than 2.5 mm. The golf ball 2 that includes a mid layer 8 having a thickness Tm of equal to or greater than 0.3 mm has a low spin rate upon a shot with a driver. In this respect, the thickness Tm is more preferably equal to or greater than 0.5 mm and particularly preferably equal to or greater than 0.8 mm. The golf ball 2 that includes the mid layer 8 having a thickness Tm of equal to or less than 2.5 mm has excellent feel at impact. In this respect, the thickness Tm is more preferably equal to or less than 2.0 mm and particularly preferably equal to or less than 1.8 mm. The thickness Tm is measured at a position immediately below the land 14.

The golf ball 2 may include two or more mid layers 8 positioned between the outer core 6 and the cover 10. In this case, the thickness of each mid layer 8 preferably falls within the above range.

The cover 10 is the outermost layer except the mark layer and the paint layer. The cover 10 is formed from a resin composition. Examples of the base polymer of the resin composition include polyurethanes, ionomer resins, polyesters, polyamides, polyolefins, and polystyrenes. A preferable base polymer in light of controllability upon hitting with a short iron is a polyurethane. When a polyurethane and another resin are used in combination for the cover 10, the proportion of the polyurethane to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

The polyurethane has a urethane bond within the molecule. The urethane bond can be formed by reacting a polyol with a polyisocyanate. In addition to the reaction for the urethane bond, a chain-lengthening reaction may be carried out. The chain-lengthening reaction can be carried out by a polyamine or a polyol having a low molecular weight.

Examples of the polyurethane include

-   -   (A1) a polyurethane including a polyisocyanate component and a         high-molecular-weight polyol component,     -   (A2) a polyurethane including a polyisocyanate component, a         high-molecular-weight polyol component, and a         low-molecular-weight polyol,     -   (A3) a polyurethane including a polyisocyanate component, a         high-molecular-weight polyol component, and a polyamine         component, and     -   (A4) a polyurethane including a polyisocyanate component, a         high-molecular-weight polyol component, a low-molecular-weight         polyol component, and a polyamine component.

The polyol, as a material for the urethane bond, has a plurality of hydroxyl groups. Low-molecular-weight polyols and high-molecular-weight polyols can be used.

Examples of low-molecular-weight polyols include diols, triols, tetraols, and hexaols. Specific examples of diols include ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, dipropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2,3-dimethyl-2,3-butanediol, neopentyl glycol, pentanediol, hexanediol, heptanediol, octanediol, and 1,6-cyclohexanedimethylol. Aniline diols or bisphenol A diols may be used. Specific examples of triols include glycerin, trimethylol propane, and hexanetriol. Specific examples of tetraols include pentaerythritol and sorbitol.

Examples of high-molecular-weight polyols include polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polytetramethylene ether glycol (PTMG); condensed polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA); lactone polyester polyols such as poly-s-caprolactone (PCL); polycarbonate polyols such as polyhexamethylene carbonate; and acrylic polyols. Two or more polyols may be used in combination. In light of feel at impact of the golf ball 2 upon a shot with a driver, the high-molecular-weight polyol has a number average molecular weight of preferably equal to or greater than 400 and more preferably equal to or greater than 1000. The number average molecular weight is preferably equal to or less than 10000. Particularly preferably polyols are diols.

The polyisocyanate, as a material for the urethane bond, has two or more isocyanate groups. Examples of polyisocyanates include aromatic polyisocyanates, alicyclic polyisocyanates, and aliphatic polyisocyanates. Two or more types of polyisocyanates may be used in combination.

Examples of aromatic polyisocyanates include 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, a mixture (TDI) of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate (TMXDI), and paraphenylene diisocyanate (PPDI).

Examples of alicyclic polyisocyanates include 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), 1,3-bis(isocyanatomethyl)cyclohexane (H₆XDI), isophorone diisocyanate (IPDI), and trans-1,4-cyclohexane diisocyanate (CHDI).

One example of aliphatic polyisocyanates is hexamethylene diisocyanate (HDI).

In light of scuff resistance, aromatic polyisocyanates are preferable. In light of weather resistance, TMXDI, XDI, HDI, H₆XDI, IPDI, H₁₂MDI, and NBD are preferable, and H₁₂MDI is particularly preferable. H₁₂MDI is excellent in both scuff resistance and weather resistance.

The polyamine for the chain-lengthening reaction has two or more amino groups. Examples of polyamines include aliphatic polyamines such as ethylenediamine, propylenediamine, butylenediamine, and hexamethylenediamine; alicyclic polyamines such as isophoronediamine and piperazine; and aromatic polyamines.

In an aromatic polyamine, an amino group is bonded to an aromatic ring. The amino group may be bonded directly to the aromatic ring. The amino group may be bonded indirectly to the aromatic ring via a lower alkylene group.

Aromatic polyamines include monocyclic aromatic polyamines and polycyclic aromatic polyamines. In a monocyclic aromatic polyamine, two or more amino groups are bounded to one aromatic ring. A polycyclic aromatic polyamine has two or more aminophenyl groups. In each of the aminophenyl groups, one or more amino groups are bonded to one aromatic ring.

Examples of monocyclic aromatic polyamines include polyamines in which an amino group is bonded directly to an aromatic ring, and polyamines in which an amino group is bonded to an aromatic ring via a lower alkylene group. Specific examples of monocyclic aromatic polyamines in which an amino group is bonded directly to an aromatic ring include phenylenediamine, toluenediamine, diethyltoluenediamine, and dimethylthiotoluenediamine. Specific examples of monocyclic aromatic polyamines in which an amino group is bonded to an aromatic ring via a lower alkylene group include xylylenediamine.

Examples of polycyclic aromatic polyamines include poly(aminobenzene) in which two or more aminophenyl groups are bonded directly to each other, and polyamines in which two or more aminophenyl groups are bonded to each other via a lower alkylene group or an alkylene oxide group. Diaminodiphenylalkanes in which two aminophenyl groups are bonded to each other via a lower alkylene group are preferable, and 4,4′-diaminodiphenylmethane and derivatives thereof are particularly preferable.

The cover 10 may include a thermoplastic polyurethane, or may include a thermosetting polyurethane. In light of productivity, the thermoplastic polyurethane is preferable. The thermoplastic polyurethane includes a polyurethane component as a hard segment, and a polyester component or a polyether component as a soft segment. The thermoplastic polyurethane is flexible. The cover 10 in which the polyurethane is used has excellent scuff resistance.

Specific examples of the thermoplastic polyurethane include trade names “Elastollan NY80A”, “Elastollan NY82A”, “Elastollan NY84A”, “Elastollan NY85A”, “Elastollan NY86A”, “Elastollan NY88A”, “Elastollan NY90A”, “Elastollan NY92A”, “Elastollan NY95A”, “Elastollan NY97A”, “Elastollan NY585”, “Elastollan KP016N”, and “Elastollan 1190ATR”, manufactured by BASF Japan Ltd.; and trade names “RESAMINE P4585LS” and “RESAMINE PS62490”, manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

The resin composition of the cover 10 may include a coloring agent, a filler, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like in an adequate amount. When the hue of the golf ball 2 is white, a typical coloring agent is titanium dioxide.

In light of durability of the cover 10, the cover 10 has a Shore D hardness Hc of preferably equal to or greater than 15, more preferably equal to or greater than 18, and particularly preferably equal to or greater than 20. In light of controllability of golf ball 2, the hardness Hc is preferably equal to or less than 40, more preferably equal to or less than 37, and particularly preferably equal to or less than 34.

The hardness Hc of the cover 10 is measured according to the standards of “ASTM-D 2240-68”. The hardness Hc is measured with a Shore D type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). For the measurement, a sheet that is formed by hot press, is formed from the same material as that of the cover 10, and has a thickness of about 2 mm is used. Prior to the measurement, a sheet is kept at 23° C. for two weeks. At the measurement, three sheets are stacked.

In light of controllability, the cover 10 has a thickness Tc of preferably equal to or greater than 0.1 mm, more preferably equal to or greater than 0.3 mm, and particularly preferably equal to or greater than 0.4 mm. In light of spin suppression upon a shot with a driver, the thickness Tc is preferably equal to or less than 2.0 mm, more preferably equal to or less than 1.5 mm, and particularly preferably equal to or less than 1.0 mm. The thickness Tc is measured at a position immediately below the land 14.

For forming the cover 10, known methods such as injection molding, compression molding, and the like can be used. When forming the cover 10, the dimples 12 are formed by pimples formed on the cavity face of a mold.

The golf ball 2 may include a reinforcing layer between the mid layer 8 and the cover 10. The reinforcing layer firmly adheres to the mid layer 8 and also to the cover 10. The reinforcing layer suppresses separation of the mid layer 8 from the cover 10. The reinforcing layer is formed from a resin composition. Examples of a preferable base polymer of the reinforcing layer include two-component curing type epoxy resins and two-component curing type urethane resins.

The golf ball 2 preferably has an amount of compressive deformation Sb of equal to or greater than 2.0 mm but equal to or less than 3.5 mm. The golf ball 2 having an amount of compressive deformation Sb of equal to or greater than 2.0 mm has excellent feel at impact. In this respect, the amount of compressive deformation Sb is preferably equal to or greater than 2.2 mm and particularly preferably equal to or greater than 2.3 mm. The golf ball 2 having an amount of compressive deformation Sb of equal to or less than 3.5 mm has a high initial speed upon a shot with a driver. In this respect, the amount of compressive deformation Sb is more preferably equal to or less than 3.2 mm and particularly preferably equal to or less than 3.0 mm.

For measurement of the amount of compressive deformation Sb, a YAMADA type compression tester (SCH) is used. In the tester, the golf ball 2 is placed on a hard plate made of metal. Next, a cylinder made of metal gradually descends toward the golf ball 2. The golf ball 2, squeezed between the bottom face of the cylinder and the hard plate, becomes deformed. A migration distance of the cylinder, starting from the state in which an initial load of 98 N is applied to the golf ball 2 up to the state in which a final load of 1274 N is applied thereto, is measured. A moving speed of the cylinder until the initial load is applied is 0.83 mm/s. A moving speed of the cylinder after the initial load is applied until the final load is applied is 1.67 mm/s.

In the golf ball 2, a value Vb calculated by the following mathematical formula exceeds 620 and is less than 900.

Vb=Tc*Hc*Hm/Tm

In other words, the golf ball 2 meets the following mathematical formula (1.4).

620<Tc*Hc*Hm/Tm<900  (1.4)

According to the finding by the present inventor, with the golf ball 2 having a value Vb of exceeding 620 and less than 900, both desired flight performance and desired controllability are achieved. In this respect, the golf ball 2 more preferably meets the following mathematical formula.

640<Tc*Hc*Hm/Tm<800

In the golf ball 2 that includes the cover 10 having a low hardness Hc and a small thickness Tc, the mathematical formula (1.4) can be met.

As shown in FIGS. 2 and 3, the contour of each dimple 12 is circular. The golf ball 2 has dimples A each having a diameter of 4.60 mm; dimples B each having a diameter of 4.50 mm; dimples C each having a diameter of 4.35 mm; dimples D each having a diameter of 4.00 mm; and dimples E each having a diameter of 3.00 mm. The number of types of the dimples 12 is five. The golf ball 2 may have non-circular dimples instead of the circular dimples 12 or together with circular dimples 12.

The number of the dimples A is 24; the number of the dimples B is 12; the number of the dimples C is 252; the number of the dimples D is 24; and the number of the dimples E is 12. The total number of the dimples 12 is 324. A dimple pattern is formed by these dimples 12 and the land 14.

FIG. 4 shows a cross section of the golf ball 2 along a plane passing through the central point of the dimple 12 and the central point of the golf ball 2. In FIG. 4, the top-to-bottom direction is the depth direction of the dimple 12. In FIG. 4, a chain double-dashed line 16 indicates a phantom sphere. The surface of the phantom sphere 16 is the surface of the golf ball 2 when it is postulated that no dimple 12 exists. The diameter of the phantom sphere 16 is equal to the diameter of the golf ball 2. The dimple 12 is recessed from the surface of the phantom sphere 16. The land 14 coincides with the surface of the phantom sphere 16. In the present embodiment, the cross-sectional shape of each dimple 12 is substantially a circular arc.

In FIG. 4, an arrow Dm indicates the diameter of the dimple 12. The diameter Dm is the distance between two tangent points Ed appearing on a tangent line Tg that is drawn tangent to the far opposite ends of the dimple 12. Each tangent point Ed is also the edge of the dimple 12. The edge Ed defines the contour of the dimple 12. In FIG. 4, a double ended arrow Dp1 indicates a first depth of the dimple 12. The first depth Dp1 is the distance between the deepest part of the dimple 12 and the surface of the phantom sphere 16. In FIG. 4, a double ended arrow Dp2 indicates a second depth of the dimple 12. The second depth Dp2 is the distance between the deepest part of the dimple 12 and the tangent line Tg.

The diameter Dm of each dimple 12 is preferably equal to or greater than 2.0 mm but equal to or less than 6.0 mm. The dimple 12 having a diameter Dm of equal to or greater than 2.0 mm contributes to flight performance upon a shot with a driver. In this respect, the diameter Dm is more preferably equal to or greater than 2.5 mm and particularly preferably equal to or greater than 2.8 mm. The dimple 12 having a diameter Dm of equal to or less than 6.0 mm does not impair a fundamental feature of the golf ball 2 being substantially a sphere. In this respect, the diameter Dm is more preferably equal to or less than 5.5 mm and particularly preferably equal to or less than 5.0 mm.

In the case of a non-circular dimple, a circular dimple having the same area as that of the non-circular dimple is assumed. The diameter of the assumed dimple can be regarded as the diameter of the non-circular dimple.

The ratio Pd of the diameter Dm of each dimple 12 relative to the diameter of the golf ball 2 is preferably equal to or greater than 9.60% but equal to or less than 10.37%. The dimple 12 having a ratio Pd of equal to or greater than 9.60% contributes to turbulization. The golf ball 2 having the dimple 12 has excellent flight performance upon a shot with a driver. In this respect, the ratio Pd is more preferably equal to or greater than 9.90% and particularly preferably equal to or greater than 10.10%. The dimple 12 having a ratio Pd of equal to or less than 10.37% does not impair a fundamental feature of the golf ball 2 being substantially a sphere. In this respect, the ratio Pd is more preferably equal to or less than 10.32% and particularly preferably equal to or less than 10.27%. In the golf ball 2 shown in FIGS. 2 and 3, each dimple C corresponds to the “dimple having a radio Pd of equal to or greater than 9.60% but equal to or less than 10.37%”.

The ratio Rs of the number of the dimples 12 each having a ratio Pd of equal to or greater than 9.60% but equal to or less than 10.37%, relative to the total number of the dimples 12, is preferably equal to or greater than 50%. The dimple pattern having a ratio Rs of equal to or greater than 50% contributes to flight performance upon a shot with a driver. In this respect, the ratio Rs is more preferably equal to or greater than 60% and particularly preferably equal to or greater than 70%. The ratio Rs may be 100%. In the golf ball 2 shown in FIGS. 2 and 3, the number of the dimples C is 252, and the total number of the dimples 12 is 324. Therefore, the ratio Rs is 77.8%.

The ratio of the number of the dimples 12 each having a ratio Pd of less than 9.60, relative to the total number of the dimples 12, is preferably less than 50%. The dimple pattern in which this ratio is less than 50% contributes to turbulization. In this respect, this ratio is more preferably equal to or less than 30% and particularly preferably equal to or less than 10%. This ratio may be zero.

The ratio of the number of the dimples 12 each having a ratio Pd of exceeding 10.37%, relative to the total number of the dimples 12, is preferably less than 50%. With the dimple pattern in which this ratio is less than 50%, the degree of freedom in designing a dimple pattern is high, and therefore the width of the land 14 is less likely to be excessively large. In this respect, this ratio is more preferably equal to or less than 30% and particularly preferably equal to or less than 10%. This ratio may be zero.

The ratio Rs' of the number of the dimples 12 each having a ratio Pd of equal to or greater than 10.10% but equal to or less than 10.37%, relative to the total number of the dimples 12, is preferably equal to or greater than 50%. The dimple pattern having a ratio Rs' of equal to or greater than 50% contributes to flight performance upon a shot with a driver. In this respect, the ratio Rs' is more preferably equal to or greater than 60% and particularly preferably equal to or greater than 70%. The ratio Rs' may be 100%. In the golf ball 2 shown in FIGS. 2 and 3, each dimple C corresponds to the “dimple having a ratio Pd of equal to or greater than 10.10% but equal to or less than 10.37%”. In the golf ball 2, the number of the dimples C is 252, and the total number of the dimples 12 is 324. Therefore, the ratio Rs' is 77.8%.

In light of suppression of rising of the golf ball 2 during flight, the first depth Dp1 of each dimple 12 is preferably equal to or greater than 0.10 mm, more preferably equal to or greater than 0.13 mm, and particularly preferably equal to or greater than 0.15 mm. In light of suppression of dropping of the golf ball 2 during flight, the first depth Dp1 is preferably equal to or less than 0.65 mm, more preferably equal to or less than 0.60 mm, and particularly preferably equal to or less than 0.55 mm.

The area S of the dimple 12 is the area of a region surrounded by the contour line of the dimple 12 when the central point of the golf ball 2 is viewed at infinity. In the case of a circular dimple 12, the area S is calculated by the following mathematical formula.

S=(Dm/2)²*Π

In the golf ball 2 shown in FIGS. 2 and 3, the area of each dimple A is 16.62 mm²; the area of each dimple B is 15.90 mm²; the area of each dimple C is 14.86 mm²; the area of each dimple D is 12.57 mm²; and the area of each dimple E is 7.07 mm².

In the present invention, the ratio of the sum of the areas S of all the dimples 12 relative to the surface area of the phantom sphere 16 is referred to as an occupation ratio So. In light of flight performance upon a shot with a driver, the occupation ratio So is preferably equal to or greater than 81.0%, more preferably equal to or greater than 82.0%, and particularly preferably equal to or greater than 83.0%. The occupation ratio So is preferably equal to or less than 95%. In the golf ball 2 shown in FIGS. 2 and 3, the total area of the dimples 12 is 4721.1 mm². The surface area of the phantom sphere 16 of the golf ball 2 is 5728.0 mm², so that the occupation ratio So is 82.4%.

From the standpoint that a sufficient occupation ratio is achieved, the total number N of the dimples 12 is preferably equal to or greater than 250, more preferably equal to or greater than 280, and particularly preferably equal to or greater than 300. From the standpoint that each dimple 12 can contribute to turbulization, the total number N of the dimples 12 is preferably equal to or less than 450, more preferably equal to or less than 400, and particularly preferably equal to or less than 380.

In the present invention, the “volume of the dimple” means the volume of a portion surrounded by the surface of the phantom sphere 16 and the surface of the dimple 12. In light of suppression of rising of the golf ball 2 during flight, the total volume of all the dimples 12 is preferably equal to or greater than 450 mm³, more preferably equal to or greater than 480 mm³, and particularly preferably equal to or greater than 500 mm³. In light of suppression of dropping of the golf ball 2 during flight, the total volume is preferably equal to or less than 750 mm³, more preferably equal to or less than 730 mm³, and particularly preferably equal to or less than 710 mm³.

In a graph shown in FIG. 5, the horizontal axis indicates the occupation ratio So of the dimples 12. In this graph, the vertical axis indicates the ratio Rs of the number of the dimples 12 each having a ratio Pd of equal to or greater than 9.60% but equal to or less than 10.37%, relative to the total number of the dimples 12. A straight line indicated by reference sign L1 in this graph is represented by the following mathematical formula.

Rs=−2.5*So+273

The golf ball 2 that belongs to the zone above the straight line L1 in this graph meets the following mathematical formula (2.1).

Rs≧−2.5*So+273  (2.1)

With the golf ball 2 that meets the mathematical formula (2.1), turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

A straight line indicated by reference sign L2 in the graph of FIG. 5 is represented by the following mathematical formula.

Rs=−2.5*So+278

The golf ball 2 that belongs to the zone above the straight line L2 in this graph meets the following mathematical formula (2.2).

Rs≧−2.5*So+278  (2.2)

With the golf ball 2 that meets the mathematical formula (2.2), turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

A straight line indicated by reference sign L3 in the graph of FIG. 5 is represented by the following mathematical formula.

Rs=−2.5*So+283

The golf ball 2 that belongs to the zone above the straight line L3 in this graph meets the following mathematical formula (2.3).

Rs≧−2.5*So+283  (2.3)

With the golf ball 2 that meets the mathematical formula (2.3), turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

In a graph shown in FIG. 6, the horizontal axis indicates the occupation ratio So of the dimples 12. In this graph, the vertical axis indicates the ratio Rs' of the number of the dimples 12 each having a ratio Pd of equal to or greater than 10.10% but equal to or less than 10.37%, relative to the total number of the dimples 12. A straight line indicated by reference sign L4 in this graph is represented by the following mathematical formula.

Rs'=−2.2*So+245

The golf ball 2 that belongs to the zone above the straight line L4 in this graph meets the following mathematical formula (2.4).

Rs′≧−2.2*So+245  (2.4)

With the golf ball 2 that meets the mathematical formula (2.4), turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

A straight line indicated by reference sign L5 in the graph of FIG. 6 is represented by the following mathematical formula.

Rs'=−2.2*So+252

The golf ball 2 that belongs to the zone above the straight line L5 in this graph meets the following mathematical formula (2.5).

Rs′≧−2.2*So+252  (2.5)

With the golf ball 2 that meets the mathematical formula (2.5), turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

As shown in FIG. 3, the surface of the golf ball 2 (or the phantom sphere 16) can be divided into two hemispheres HE by an equator Eq. Specifically, the surface can be divided into a northern hemisphere NH and a southern hemisphere SH. Each hemisphere HE has a pole P. The pole P corresponds to a deepest point of a mold for the golf ball 2.

FIG. 2 shows the northern hemisphere. The southern hemisphere has a pattern obtained by rotating the dimple pattern in FIG. 2 about the pole P. Line segments S1, S2, and S3 shown in FIG. 2 each extend from the pole P. The angle at the pole P between the line segment S1 and the line segment S2 is 120°. The angle at the pole P between the line segment S2 and the line segment S3 is 120°. The angle at the pole P between the line segment S3 and the line segment S1 is 120°.

Of the surface of the golf ball 2 (or the phantom sphere 16), a zone surrounded by the line segment S1, the line segment S2, and the equator Eq is a first spherical triangle T1. Of the surface of the golf ball 2 (or the phantom sphere 16), a zone surrounded by the line segment S2, the line segment S3, and the equator Eq is a second spherical triangle T2. Of the surface of the golf ball 2 (or the phantom sphere 16), a zone surrounded by the line segment S3, the line segment S1, and the equator Eq is a third spherical triangle T3. Each spherical triangle is a unit. The hemisphere HE can be divided into the three units.

When the dimple pattern of the first spherical triangle T1 is rotated by 120° about a straight line connecting the two poles P, the resultant dimple pattern substantially overlaps the dimple pattern of the second spherical triangle T2. When the dimple pattern of the second spherical triangle T2 is rotated by 120° about the straight line connecting the two poles P, the resultant dimple pattern substantially overlaps the dimple pattern of the third spherical triangle T3. When the dimple pattern of the third spherical triangle T3 is rotated by 120° about the straight line connecting the two poles P, the resultant dimple pattern substantially overlaps the dimple pattern of the first spherical triangle T1. In other words, the dimple pattern of the hemisphere is composed of three units that are rotationally symmetrical to each other.

A pattern obtained by rotating the dimple pattern of each hemisphere HE by 120° about the straight line connecting the two poles P substantially overlaps the dimple pattern that has not been rotated. The dimple pattern of each hemisphere HE has 120° rotational symmetry.

A line segment S4 shown in FIG. 2 extends from the pole P. The angle at the pole P between the line segment S4 and the line segment S1 is 60°. The angle at the pole P between the line segment S4 and the line segment S2 is 60°. The first spherical triangle T1 (unit) can be divided into a small spherical triangle T1 a and a small spherical triangle T1 b by the line segment S4. The spherical triangle T1 a and the spherical triangle T1 b are small units.

A pattern obtained by inverting the dimple pattern of the spherical triangle T1 a with respect to a plane containing the line segment S4 and the straight line connecting both poles P substantially overlaps the dimple pattern of the spherical triangle T1 b. In other words, the dimple pattern of the spherical triangle T1 a (unit) is composed of two small units that are mirror-symmetrical to each other.

Similarly, the dimple pattern of the second spherical triangle T2 is also composed of two small units that are mirror-symmetrical to each other. The dimple pattern of the third spherical triangle T3 is also composed of two small units that are mirror-symmetrical to each other. The dimple pattern of the hemisphere HE is composed of the six small units.

According to the finding by the present inventor, with the golf ball 2 of which the dimple pattern of each hemisphere is composed of three units that are rotationally symmetrical to each other by 120° and the dimple pattern of each unit is composed of two small units that are mirror-symmetrical to each other, turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

EXAMPLES Example 1

A rubber composition (b) was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 26.0 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.5 parts by weight of diphenyl disulfide, and 0.7 parts by weight of dicumyl peroxide. This rubber composition (b) was placed into a mold that includes upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 15 minutes to obtain an inner core with a diameter D1 of 24 mm. The amount of barium sulfate was adjusted such that a predetermined ball weight is obtained.

A rubber composition (d) was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 41.5 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.1 parts by weight of an antioxidant (H-BHT), 0.5 parts by weight of diphenyl disulfide, and 0.7 parts by weight of dicumyl peroxide. Half shells were formed from this rubber composition (d). The inner core was covered with two of these half shells. The inner core and the half shells were placed into a mold that includes upper and lower mold halves each having a hemispherical cavity, and a core consisting of the inner core and an outer core was formed by a first crosslinking process and a second crosslinking process. In the first crosslinking process, the crosslinking temperature was 140° C., and the crosslinking time period was 20 minutes. In the second crosslinking process, the crosslinking temperature was 160° C., and the crosslinking time period was 10 minutes. The diameter of the core was 38.5 mm.

A resin composition (M2) was obtained by kneading 50 parts by weight of an ionomer resin (the aforementioned “Surlyn 8150”), 50 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), and 4 parts by weight of titanium dioxide with a twin-screw kneading extruder. The core was covered with this resin composition (M2) by injection molding to form a mid layer with a thickness of 1.6 mm.

A paint composition (trade name “POLIN 750LE”, manufactured by SHINTO PAINT CO., LTD.) including a two-component curing type epoxy resin as a base polymer was prepared. The base material liquid of this paint composition includes 30 parts by weight of a bisphenol A type epoxy resin and 70 parts by weight of a solvent. The curing agent liquid of this paint composition includes 40 parts by weight of a modified polyamide amine, 55 parts by weight of a solvent, and 5 parts by weight of titanium dioxide. The weight ratio of the base material liquid to the curing agent liquid is 1/1. This paint composition was applied to the surface of the mid layer with a spray gun, and kept at 23° C. for 12 hours to obtain a reinforcing layer with a thickness of 10 μm.

A resin composition (C2) was obtained by kneading 100 parts by weight of a thermoplastic polyurethane elastomer (the aforementioned “Elastollan NY84A”), 0.2 parts by weight of a light stabilizer (trade name “TINUVIN 770”), 4 parts by weight of titanium dioxide, and 0.04 parts by weight of ultramarine blue with a twin-screw kneading extruder. Half shells were obtained from this resin composition (C2) by compression molding. The sphere consisting of the inner core, the outer core, the mid layer, and the reinforcing layer was covered with two of the half shells. These half shells and the sphere were placed into a final mold that includes upper and lower mold halves each having a hemispherical cavity and having a large number of pimples on its cavity face, and a cover was obtained by compression molding. The thickness of the cover was 0.5 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the cover.

A clear paint including a two-component curing type polyurethane as a base material was applied to this cover to obtain a golf ball of Example 1 with a diameter of about 42.7 mm and a weight of about 45.6 g.

Examples 2 to 8 and Comparative Examples 1 to 6

Golf balls of Examples 2 to 8 and Comparative Examples 1 to 6 were obtained in the same manner as Example 1, except the specifications of the inner core, the outer core, the mid layer, the cover, and the dimples were as shown in Tables 7 to 9 below. The specifications of the inner core and the outer core are shown in detail in Tables 1 and 2 below. The specifications of the mid layer are shown in detail in Table 3 below. The specifications of the cover are shown in detail in Table 4 below. The specifications of the dimples are shown in detail in Tables 5 and 6 below.

[Flight Test]

A driver (trade name “Z745”, manufactured by DUNLOP SPORTS CO. LTD., shaft hardness: S, loft angle: 8.5°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under a condition of a head speed of 50 m/sec, and the initial speed, the spin rate, and the flight distance of the golf ball were measured. The flight distance is the distance between the point at the hit and the point at which the ball stopped. The average value of data obtained by 12 measurements is shown in Tables 7 to 9 below.

TABLE 1 Composition of Core (parts by weight) a b c d Polybutadiene 100    100    100    100    Zinc diacrylate — 26.0  31.5  41.5  Magnesium oxide 35   — — — Methacrylic acid 28   — — — Zinc oxide — 5   5   5   Barium sulfate * * * * H-BHT — — — 0.1 2-naphthalenethiol — — 0.2 — Diphenyl disulfide — 0.5 — 0.5 Pentabromodiphenyl disulfide — — 0.3 — Dicumyl peroxide 0.9 0.7 0.8 0.7 * Appropriate amount

TABLE 2 Specifications of Core I II III IV Inner Composition a b b b layer Crosslinking 170 170 170 170 temperature (° C.) Crosslinking time 20 15 15 15 period (min) Diameter D1 (mm) 15 24 27 20 Outer Composition c d d d layer First crosslinking 150 140 140 140 temperature (° C.) First crosslinking 20 20 20 20 time period (min) Second crosslinking — 160 160 160 temperature (° C.) Second crosslinking — 10 10 10 time period (min)

TABLE 3 Specifications of Mid Layer (parts by weight) M1 M2 M3 M4 Surlyn 8150 — 50 25 32.5 Surlyn 9150 — — — 32.5 Polyamide 6 — — — 35 Himilan 1605 47 — 25 — Himilan AM7329 50 50 50 — RABALON T3221C 3 — — — Titanium dioxide 4 4 4 4 Hardness Hm (Shore D) 63 68 66 72

TABLE 4 Specifications of Cover (parts by weight) C1 C2 C3 Elastollan NY82A 100 — — Elastollan NY84A — 100 — Elastollan NY90A — — 100 TINUVIN 770 0.2 0.2 0.2 Titanium dioxide 4 4 4 Ultramarine blue 0.04 0.04 0.04 Hardness Hc (Shore D) 29 31 38

TABLE 5 Specifications of Dimples Num- Dm Dp2 Dp1 R Volume Pd ber (mm) (mm) (mm) (mm) (mm³) (%) D1 A 24 4.60 0.135 0.2592 19.66 1.123 10.77 B 12 4.50 0.135 0.2539 18.82 1.075 10.54 C 252 4.35 0.135 0.2461 17.59 1.004 10.19 D 24 4.00 0.135 0.2289 14.88 0.850 9.37 E 12 3.00 0.135 0.1878 8.40 0.478 7.03 D2 A 24 4.60 0.135 0.2592 19.66 1.123 10.77 B 54 4.50 0.135 0.2539 18.82 1.075 10.54 C 210 4.40 0.135 0.2487 17.99 1.028 10.30 D 24 4.00 0.135 0.2289 14.88 0.850 9.37 E 12 3.00 0.135 0.1878 8.40 0.478 7.03 D3 A 24 4.60 0.135 0.2592 19.66 1.123 10.77 B 12 4.50 0.135 0.2539 18.82 1.075 10.54 C 210 4.35 0.135 0.2461 17.59 1.004 10.19 D 66 4.05 0.135 0.2313 15.26 0.871 9.48 E 12 3.00 0.135 0.1878 8.40 0.478 7.03

TABLE 6 Specifications of Dimples D1 D2 D3 Plan view FIG. 2 FIG. 7 FIG. 9 Front view FIG. 3 FIG. 8 FIG. 10 Number 324 324 324 Number of units 3 3 3 Number of small units 6 6 6 So (%) 82.4 84.4 81.1 Rs (%) 77.8 64.8 64.8 Rs + 2.5 * So − 273 10.80 2.80 −5.45 Mathematical formula (1) Met Met Unmet Rs + 2.5 * So − 278 5.80 −2.20 −10.45 Mathematical formula (2) Met Unmet Unmet Rs + 2.5 * So − 283 0.80 −7.20 −15.45 Mathematical formula (3) Met Unmet Unmet Rs′ (%) 77.8 64.8 64.8 Rs′ + 2.2 * So − 245 14.08 5.48 −1.78 Mathematical formula (4) Met Met Unmet Rs′ + 2.2 * So − 252 7.08 −1.52 −8.78 Mathematical formula (5) Met Unmet Unmet

TABLE 7 Results of Evaluation Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 1 Ex. 4 Core II II II II III Inner core D1 (mm) 24.0 24.0 24.0 24.0 27.0 V1 (mm³) 7238 7238 7238 7238 10306 H1o (Shore C) 60 60 60 60 60 H1in (Shore C) 74 74 74 74 74 De1 = H1in − H1o 14 14 14 14 14 Outer core D2 (mm) 38.5 38.5 39.1 38.1 38.5 V2 (mm³) 22642 22642 24061 21720 19574 H2out (Shore C) 82 82 82 82 82 H2s (Shore C) 82 82 82 82 82 De2 = H2s − H2out 0 0 0 0 0 Mid layer M2 M2 M2 M2 M2 Hm (Shore D) 68 68 68 68 68 Tm (mm) 1.6 1.6 1.3 1.8 1.6 Cover C2 C2 C2 C2 C2 Hc (Shore D) 31 31 31 31 31 Tc (mm) 0.5 0.5 0.5 0.5 0.5 Dimple D1 D2 D1 D1 D1 V2/V1 3.1 3.1 3.3 3.0 1.9 De2 − De1 −14 −14 −14 −14 −14 Va 804 804 804 804 905 Vb 659 659 811 586 659 Sb (mm) 2.3 2.3 2.4 2.2 2.3 Initial speed (m/s) 73.7 73.7 73.5 73.8 73.6 Spin (rpm) 2500 2500 2400 2700 2500 Distance (yd) 290.0 289.5 290.0 288.5 289.5

TABLE 8 Results of Evaluation Comp. Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 2 Core IV II II II II Inner core D1 (mm) 20.0 24.0 24.0 24.0 24.0 V1 (mm³) 4189 7238 7238 7238 7238 H1o (Shore C) 60 60 60 60 60 H1in (Shore C) 74 74 74 74 74 De1 = H1in − H1o 14 14 14 14 14 Outer core D2 (mm) 38.5 38.5 38.5 38.5 38.5 V2 (mm³) 25691 22642 22642 22642 22642 H2out (Shore C) 82 82 82 82 82 H2s (Shore C) 82 82 82 82 82 De2 = H2s − H2out 0 0 0 0 0 Mid layer M2 M3 M4 M2 M2 Hm (Shore D) 68 66 72 68 68 Tm (mm) 1.6 1.6 1.6 1.6 1.6 Cover C2 C2 C2 C3 C1 Hc (Shore D) 31 31 31 38 29 Tc (mm) 0.5 0.5 0.5 0.5 0.5 Dimple D1 D1 D1 D1 D1 V2/V1 6.1 3.1 3.1 3.1 3.1 De2 − De1 −14 −14 −14 −14 −14 Va 670 804 804 804 804 Vb 659 639 698 808 616 Sb (mm) 2.3 2.3 2.2 2.3 2.3 Initial speed (m/s) 73.7 73.7 73.7 73.7 73.6 Spin (rpm) 2550 2550 2450 2350 2600 Distance (yd) 289.5 289.5 290.5 291.5 288.5

TABLE 9 Results of Evaluation Comp. Comp. Comp. Comp. Ex. 9 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Core II II II II I Inner core D1 (mm) 24.0 24.0 24.0 24.0 15.0 V1 (mm³) 7238 7238 7238 7238 1767 H1o (Shore C) 60 60 60 60 62 H1in (Shore C) 74 74 74 74 65 De1 = H1in − H1o 14 14 14 14 3 Outer core D2 (mm) 38.3 38.5 39.7 38.5 39.7 V2 (mm³) 22179 22642 25524 22642 30995 H2out (Shore C) 82 82 82 82 71 H2s (Shore C) 82 82 82 82 85 De2 = H2s − H2out 0 0 0 0 14 Mid layer M2 M2 M2 M1 M2 Hm (Shore D) 68 68 68 63 68 Tm (mm) 1.6 1.6 1.0 1.6 1.0 Cover C2 C2 C2 C2 C1 Hc (Shore D) 31 31 31 31 29 Tc (mm) 0.6 0.5 0.5 0.5 0.5 Dimple D1 D3 D1 D1 D1 V2/V1 3.1 3.1 3.5 3.1 17.5 De2 − De1 −14 −14 −14 −14 11 Va 804 804 804 804 476 Vb 791 659 1054 610 986 Sb (mm) 2.3 2.3 2.5 2.4 2.3 Initial speed (m/s) 73.7 73.7 73.2 73.3 73.4 Spin (rpm) 2600 2500 2400 2550 2500 Distance (yd) 289.0 288.5 288.5 287.5 288.5

As shown in Tables 7 to 9, the golf ball of each Example has excellent flight performance upon a shot with a driver. From the results of evaluation, advantages of the present invention are clear.

The golf ball according to the present invention is suitable for, for example, playing golf on golf courses and practicing at driving ranges. The above descriptions are merely illustrative examples, and various modifications can be made without departing from the principles of the present invention. 

What is claimed is:
 1. A golf ball comprising an inner core, an outer core positioned outside the inner core, a mid layer positioned outside the outer core, and a cover positioned outside the mid layer, wherein the golf ball has a plurality of dimples on a surface thereof, a ratio So of a sum of areas of the dimples relative to a surface area of a phantom sphere of the golf ball is equal to or greater than 81.0%, a ratio Rs of a number of the dimples each having a diameter of equal to or greater than 9.60% but equal to or less than 10.37%, of a diameter of the golf ball, relative to a total number of the dimples, is equal to or greater than 50%, a dimple pattern of each hemisphere of the golf ball includes three units that are rotationally symmetrical to each other, a dimple pattern of each unit includes two small units that are mirror-symmetrical to each other, and the golf ball meets the following mathematical formula (2.1): Rs≧−2.5*So+273  (2.1).
 2. The golf ball according to claim 1, wherein a volume V1 (mm³) of the inner core and a volume V2 (mm³) of the outer core meets the following mathematical formula (1.1): 1.0<V2/V1<7.0  (1.1).
 3. The golf ball according to claim 2, wherein the golf ball meets the following mathematical formula: 2.0<V2/V1<6.0.
 4. The golf ball according to claim 1, wherein a difference De1 between a boundary inside hardness H1in (Shore C) and a hardness H1o (Shore C) at a central point of the inner core; and a difference De2 between a surface hardness H2s (Shore C) and a boundary outside hardness H2out (Shore C) of the outer core meets the following mathematical formula (1.2): De2−De1<0  (1.2).
 5. The golf ball according to claim 1, wherein a diameter D1 (mm), a hardness H1o (Shore C) at a central point, and a boundary inside hardness H1in (Shore C) of the inner core meets the following mathematical formula (1.3): 600<(H1o+H1in)*(D1/2)/2<1000  (1.3).
 6. The golf ball according to claim 5, wherein the golf ball meets the following mathematical formula: 700<(H1o+H1in)*(D1/2)/2<900.
 7. The golf ball according to claim 1, wherein a thickness Tm (mm) and a hardness Hm (Shore D) of the mid layer; and a thickness Tc (mm) and a hardness Hc (Shore D) of the cover meets the following mathematical formula (1.4): 620<Tc*Hc*Hm/Tm<900  (1.4).
 8. The golf ball according to claim 7, wherein the golf ball meets the following mathematical formula: 640<Tc*Hc*Hm/Tm<800.
 9. The golf ball according to claim 1, wherein the golf ball meets the following mathematical formula (2.2): Rs≧−2.5*So+278  (2.2).
 10. The golf ball according to claim 9, wherein the golf ball meets the following mathematical formula (2.3): Rs≧−2.5*So+283  (2.3).
 11. The golf ball according to claim 1, wherein a ratio Rs' of a number of the dimples each having a diameter of equal to or greater than 10.10% but equal to or less than 10.37%, of the diameter of the golf ball, relative to the total number of the dimples, is equal to or greater than 50%, and the golf ball meets the following mathematical formula (2.4): Rs′≧−2.2*So+245  (2.4).
 12. The golf ball according to claim 11, wherein the golf ball meets the following mathematical formula (2.5): Rs′≧−2.2*So+252  (2.5).
 13. The golf ball according to claim 1, wherein a depth of a deepest part of each dimple from a surface of the phantom sphere is equal to or greater than 0.10 mm but equal to or less than 0.65 mm.
 14. The golf ball according to claim 1, wherein a total volume of the dimples is equal to or greater than 450 mm³ but equal to or less than 750 mm³. 